1. Field of the Invention
This invention relates to a firedamp-safe propellant charge. More especially, this invention relates to a coated propellant charge which is firedamp-safe. This invention is particularly concerned with firedamp-safe propellant charges employed in connection with stud driving cartridges. These propellant charges are therefore operable to drive studs in mines and the like where otherwise there would be fire and/or explosion hazards. With the propellant charges of the invention, for the first time, stud setting machines equipped with propellant charges can be used in coal mining operations.
2. Discussion of the Prior Art
Stud setting machines operate by firing a propellant cartridge in a cartridge chamber. The energy thus liberated drives a piston or plunger, to the front end of which a steel stud is fastened in a socket. This stud is then driven with great force into iron, concrete, masonry or other building materials. Lastly, the excess pressure of the propellant gases is released through a relief aperture.
In mining opeations it has not been possible hitherto to use stud setting machines in atmosphere containing firedamp. Propellant cartridges for these machines contain as propellants either pyrotechnical mixtures or a powder charge which releases the energy and converts it to power in the machine. To produce this power, the explosion heat or the combustion energy of the propellant must be proportionately high. It follows that the explosion temperature must also be high. This explosion temperature, however, is so high that the release of power is regularly accompanied by fire or flash. Even if the flash is externally invisible and the gases are greatly cooled by carrying them through a labyrinth until they emerge into the open, it must be anticipated, in the case of a normal machine, that firedamp gases will penetrate into the machine and can there be ignited. In other words, the ignition of an explosive gas mixture must be anticipated.
The tendency towards firedamp ignition can be simulated by the following experiment: a propellant cartridge is fired in a firing device other than a stud setter into an explosive gas atmosphere which is contained, for example, in a thin gas balloon placed in front of the cartridge. The gas atmosphere is regularly ignited when the propellants known hitherto are fired.
The possibilities of improving the propellant charges such that ignition will be prevented are very slight. The space available in the charge chamber of the cartridge is limited. In addition, charges which are commonly used to conceal muzzle flash cannot be used because they will too quickly foul the machine.
The reduction of the specific energy of the charge and with it the explosion temperature either by changing the charge components or by the admixture of energy reducing substances will not bring the desired success, because then the necessary power is not achieved, which is expressed, for example, as the dynamic liveliness: ##EQU1## Thus, it is no longer possible for a rapidly effective thrust to develop. This is because an only slightly extended burn time reduces the acceleration of the stud to such an extent that it is no longer driven into the building material.
It is an object of this invention, therefore, to provide a propellant charge for a cartridge of a stud setting machine which does not cause ignition of firedamp mixtures. More especially, it is an object of this invention to provide such a propellant charge which, while preventing ignition of a firedamp mixture, retains sufficient power to effect setting of the stud. It is a further object of this invention to provide a coated propellant charge which, when employed in a stud setting machine, inserts a stud in a coal mine without effecting ignition of a firedamp mixture therein.